[MEDIUM] stats: add the ability to enable/disable/shutdown a frontend at runtime

[MEDIUM] stats: add the ability to enable/disable/shutdown a frontend at runtime

The stats socket now allows the admin to disable, enable or shutdown a frontend.
This can be used when a bug is discovered in a configuration and it's desirable
to fix it but the rules in place don't allow to change a running config. Thus it
becomes possible to kill the frontend to release the port and start a new one in
a separate process.

This can also be used to temporarily make haproxy return TCP resets to incoming
requests to pretend the service is not bound. For instance, this may be useful
to quickly flush a very deep SYN backlog.

The frontend check and lookup code was factored with the "set maxconn" usage.